In aircraft construction, panels for the fuselage construction, for example, can be provided as laminate panels where a number of metal sheets are attached to each other by providing fibre reinforced adhesive layers in between adjacent metal sheets. However, due to the use of metal foils and their given maximum size, the laminates are restricted to a maximum width, depending on the width of the provided metal foils. Hence, for larger fuselage construction segments, for example, riveted joints between adjacent foils become necessary for forming larger panels. As a result of an increasing number of joints, the weight of the fuselage construction increases, meaning a decrease in terms of the aircraft's efficiency during operation. Therefore, splicing is used, wherein adjacent metal foils are connected via a splice connection in order to form larger panels, compared to the size of the individual metal sheets. For example, U.S. Pat. No. 5,429,326 describes splicing of adjacent metal foils by an abutting arrangement, wherein the abutment joints are displaced in the direction of the cross-section. Further, U.S. Pat. No. 6,736,919 describes a splice connection, in which adjacent sheets are at least partly overlapping at their edge zones. However, it has been shown that an area comprising resin or matrix material is provided in an exposed manner on the outside of the panel, thus resulting in a zone of reduced strength.
Hence, there may be a need to provide a laminate construction with increased size and improved performance, in particular, to achieve an increased life performance of the construction.
According to an embodiment, an aircraft panel with a laminate structure is provided, wherein the laminate structure comprises a stack of a plurality of metal sheet layers, at least one fibre reinforced adhesive layer, and at least one cover segment. At least one outer layer of the metal sheet layers comprises at least two separate metal sheets that overlap with each other along their respective commonly adjoining edges, providing an overlapping joint of the two separate metal foils. The at least one fibre reinforced adhesive layer comprises fibre elements embedded in a matrix structure. One of the at least one fibre reinforced adhesive layers is arranged between two adjacent metal sheet layers. The at least one cover segment is arranged on an outside surface of the laminate structure. The cover segment covers a region of the overlapping joint. The at least one cover segment comprises at least one layer of reinforcement fibres.
The term “a region” of the overlapping joint relates to the projection of two overlapping edge portions of the two metal sheets, plus an adjacent area providing a transition zone or transition region from the edge of the overlapping metal sheet to the outer surface of the other metal sheet.
The cover segment is provided as a separate sheet, separate from the metal sheets. In an example, the cover segment is made from a different material as are the metal sheets.
In one embodiment, the aircraft panel is provided as a fuselage panel. In another embodiment, the aircraft panel is provided as a wing panel, in a further embodiment, the aircraft panel is provided as an empennage panel.
In another embodiment, one (or more) single shear joint(s) of the two separate metal foils is (are) provided. In another embodiment, alternatively or in addition, one (or several) double shear joint(s) is (are) provided.
The term “outer layer” relates to a layer arranged on the outside of a stack of layers; i.e the term “outer layer” relates to the first or last layer, depending on where the layers of the respective stack are counted from.
In a further embodiment, when used for a fuselage structure of an aircraft, the term “outer layer” relates to a layer arranged on the outside of the aircraft structure.
In one embodiment, the at least one cover segment; e.g., one cover segment, two cover segments, three cover segments, four cover segments, five cover segments or more cover segments, is arranged on only one side of the aircraft panel, i.e. on one of the two outer surfaces of the laminate structure. In a further embodiment, the at least one cover segment, for example one cover segment or, as mentioned above, two or more cover segments, is arranged on both sides of the aircraft panel, i.e. on both outer surfaces of the laminate structure.
The term “a plurality of sheets” relates to at least two sheets, comprising three, four, five, six, seven, eight, nine, ten and more sheets. The commonly adjoining edges form an overlap. The overlap thus forms a load carrying sheet joint. The overlapping joint forms a splice connection of the adjacent metal foils.
An adhesive is provided between the edges of the two adjoining sheets in the overlapping region. The adhesive is provided as an adhesive connection-layer, or adhesive layer-segment. In a further example, the edges of the two adjoining sheets are connected in the overlapping region via further means, alternatively, or in addition to an adhesive, for example via bonding.
The at least one cover segment covers adjacent regions of the respective metal sheets that are connected via the overlapping joint.
The matrix structure comprises matrix material; e.g., epoxy material and/or other resin material is provided. As fibre elements that are embedded in the matrix structure, carbon fibres, glass fibres, aramid fibres, or other types of fibres for reinforcement are used.
The metal sheet layers comprise metal foil made from, for example, aluminium.
The overlapping joints may be arranged in a straight manner, i.e. when projecting the aircraft panel onto a horizontal surface, the overlapping joints form lines with linear extension. In another embodiment, the overlapping joints are arranged in a curved manner.
The overlapping provided by the overlapping joints may be arranged on one side of the metal sheets, thus extending a width, for example, in one direction. In a further embodiment, the overlapping is provided on two opposing edges of the metal sheets, for example, providing an extension of the width in two directions. In a still further embodiment, the overlapping is arranged on three or more sides of the metal sheets. The extension of the metal sheet width via the overlapping joints may be provided on further sides; e.g., on two opposing sides. In addition or alternatively, the extension is provided in the length direction, with respect to the metal sheet provided, for example, in roll segments.
For covering the resulting regions of the overlapping joints, a plurality of cover segments is provided accordingly. For example, a pattern or grid of cover segments thus may be provided.
According to an embodiment, in the region of the overlapping joint, due to the overlap of the two metal sheets, a transition region is provided, in which one of the metal sheets is arranged overlapping the other metal sheet. The transition region forms a recess, and the cover segment sealingly covers the matrix material in a groove-like recess to protect it from the environment. The recess may be a groove-like recess, for example, shaped with an approximately triangular cross-section.
According to an embodiment, in the region of the overlapping joint, due to the overlap of the two metal sheets, a transition region is provided, in which one of the metal sheets is bent such that it is arranged overlapping with the other metal sheet. The transition region forms a groove-like recess, which is at least partly filled with matrix material. The cover segment sealingly covers the matrix material in the groove-like recess from an environment.
In an example, the cover segment sealingly covers the groove-like recess such that is sealed against the environment. The cover segment thus provides a capping or closing of the matrix material to avoid the matrix material being exposed to the environment. According to a further embodiment, the groove-like recess is formed by an end face of the overlapping metal sheet arranged overlapping on the outside of the other metal sheet, and an outwards facing surface of the bent metal sheet, and a side edge zone of the overlapping joint. The cover segment is at least larger than the groove-like recess to provide a secure sealing of the groove-like recess.
According to an embodiment, in the region of the overlapping joint, the laminate is provided with a flush outer surface in the region of the overlapping joint.
The cover segment extends beyond the projection of the overlapping joint of the adjacent outer metal sheets. In an embodiment, the outer surface of the panel is flush and the metal sheets have a secondary transition zone where the cover segment ends, and wherein a secondary groove is formed. In an embodiment, the cover segment is thinner than the adjacent overlapping metal sheets and the secondary groove is smaller in its depth than the groove-like recess caused by the overlap of the metal sheets. The relative size may be larger than 1:2, e.g. 1:3 or more.
The cover segment extends beyond the projection of the overlapping joint by at least a distance of at least the thickness of the panel. In another example, the excess distance is at least the projected overlap depth, with respect to the length in a direction of the overlap.
The term “outer surface” relates to the outer contour of the panel. The term “flush” relates to a surface with a continuous or smooth contour, i.e. the contour does not reflect (or at least only in a reduced amount) an underlying overlapping joint of two adjoining panels. The underlying structure is arranged such that displacements in the extension of the panels in the different layers is equalized to an even or flush outer contour, omitting surface protrusions.
According to an embodiment, the at least one cover segment comprises a fibre/resin prepreg. According to further embodiment, the reinforcement fibre prepregs are arranged in a unidirectional orientation crosswise to the orientation of the overlapping joint. The term “prepregs” relates to pre-impregnated fibre structures, i.e. reinforcement fibres already combined with at least some matrix material for improved handling. Instead of matrix material, binder material or the like can be provided for attaching the fibres to each other for handling purposes. The matrix material may be provided in a sufficient amount in view of the curing process. Additional matrix material may be provided after placing the prepreg on the laminate structure or on a tool.
The reinforcement fibres are arranged in a perpendicular direction with respect to an overlapping direction of the two adjoined panels. The term “perpendicular” relates to an angle of approximately 90°, for example an angle of 80° to an angle of 100°, or preferably an angle of 85° to 95°. In another embodiment, the reinforcement fibres are arranged in a bidirectional orientation having two fibre directions, wherein the two fibre directions are arranged crosswise to each other, and wherein both fibre directions are arranged transverse to the orientation of the overlapping joint.
An adhesive may be provided to attach the cover segment to the metal sheets of the outer layer. In one example, the adhesive is a resin. In another example, the adhesive is an adhesive layer provided on the cover segment. At least one cover segment is arranged on only one side of the panel on the outside of an aircraft exposed to the environment.
According to a further embodiment, besides the at least one outer layer, further layers of the metal sheet layers comprise at least two separate metal sheets that overlap each other along their respective commonly adjoining edges, providing further overlapping joints of the two separate metal sheets. In a cross-section, the further overlapping joints are arranged in a displaced manner.
The cover segment may be provided with a smaller thickness than the metal sheet layers. In another example, the cover segment is provided with a smaller thickness than the fibre reinforced adhesive layers. In a still further example, the cover segment is provided with a similar thickness than the metal sheet layers and/or than the fibre reinforced adhesive layers. For example, for the metal sheet layers, aluminium foils of 0.3 mm are provided. In another example, the cover segment is provided with a maximum thickness of half of the metal sheet layers.
According to a further embodiment, also an aircraft is provided comprising at least a fuselage structure, a wing structure, and an empennage structure. The fuselage structure, the wing structure, and/or the empennage structure is provided with a load carrying structure and covering segments that provide aerodynamically effective surfaces. At least a part of the covering segments is provided as an aircraft panel in accordance with one of the preceding examples. The aerodynamically effective surfaces are provided by the outside surface of the fuselage panel. The term “fuselage structure” relates to the main structure of an aircraft, usually housing a cockpit, a cabin, and/or a payload area. The term “wing structure” relates to structures providing the uplift when the aircraft is travelling. The term “empennage structure” relates to stabilizing surfaces for vertical and/or horizontal stabilizing.
The “aerodynamically effective surfaces” relate to surfaces exposed to airflow during flight. The “aerodynamically effective surfaces” relate to, for example, surfaces that are exposed to the outside air during flight. The “aerodynamically effective surfaces” are provided e.g. in order to generate forces acting on the aircraft's structure, i.e. on the aerodynamically effective surfaces, such as during flight for generating uplift forces or for temporarily generating steering forces (or steering thrust) to the sides, or up or down. In other words, during flight operation, the covering segments are exposed to airflow on their respective pouter surfaces (while the inner surfaces may face towards inner parts of the aircraft, i.e. parts that are not exposed to the airflow. In addition to covering segments that are exposed to the airflow during the flight, further covering segments may be provided that are exposed to the air, or even the airflow, but without being provided for generating any forces. Further, other types of covering segments may also be provided.
The load carrying structure may be provided as a frame structure, to which the covering segments are attached, for example as the aircraft skin structure. For example, the aircraft is provided as an airplane with wings. In another example, the aircraft is provided as a helicopter.
By providing at least a part of the covering segments, for example in areas with an increased span, as aircraft panels according to the above-described examples with the particular laminate structure comprising the cover segments, increased stability of the outer structure is provided, thus providing an increased performance over the lifetime of the structure.
In accordance with a further embodiment, a method for manufacturing an aircraft panel is provided, comprising the following steps:
a) arranging at least one cover segment that comprises at least one layer of reinforcement fibres on a tool;
b) arranging on the tool at least two separate metal sheets that overlap with each other along their respective commonly adjoining edges, providing an overlapping joint of the two separate metal sheets, wherein the overlapping joint is arranged on the cover segment, and wherein the metal sheets form a metal sheet layer;c) arranging on the metal sheet layer a fibre reinforced adhesive layer, comprising fibre elements embedded in a matrix structure; andd) arranging a further metal sheet layer on the fibre reinforced adhesive layer.
In an example, the provided laminate structure is cured by applying heat and/or pressure. In a further example, the curing process is provided by curing of the matrix structure without heat and the application of pressure.
According to a further embodiment, before step a), a first tool with a flush surface contour is provided for the placing of the laminate structure. For example, after step d), a second tool is provided for applying pressure and/or heat. Steps c) and d) are repeated, thus forming a stack of a plurality of metal sheet layers arranged in an alternating manner such that one fibre reinforced adhesive layer is arranged between two adjacent metal sheet layers. According to an embodiment, a plurality of metal sheet layers comprises at least two separate metal sheets with an overlapping joint.
According to a further embodiment, also the use of an aircraft panel according to one of the above-mentioned examples in an aircraft is provided.
According to a further embodiment, a cover segment is provided bridging a zone in which a resin rich part of the panel is arranged on the outside of the panel, in particular in view of the use on a fuselage structure, where such zone would be exposed to the outside of the fuselage. The bridging is provided by components having the capacity of not only covering this region, but also of providing the transmission of tensile forces, for example prepregs, or other mat-like structures, also referred to as fibre mats. Thus, an overlapping region of a metal foil/metal foil joint is covered and thus sealed against environmental influences. Hence, by covering the zones where otherwise resin would be exposed on the outside, wherein the covering is arranged by cured prepreg fibre layer(s), the life performance of the overall construction is increased significantly.
These and other objects, features and advantages of the present embodiment will become apparent from the following detailed description taken in conjunction with the accompanying drawings.